1. Field of the Invention
The present invention relates to a method and device for direct non vaporizing on-column injection of samples containing high-boiling point and/or medium volatile compounds in gas chromatographic analysis on capillary columns.
2. Description of the Prior Art
Gas chromatography using a capillary column operates in accordance with well known principles, using an apparatus also well known, consisting essentially of an oven or gas chromatography chamber containing the capillary column; an injector device at the upstream end of the column and a detector device at the downstream end. The sample is diluted in an appropriate solvent and admitted to the column through the injector. It is carried through the column by the carrier gas. The elution of the sample occurs according to a time parameter based principally on the affinity of the sample for the stationary phase, which coats the inside of the column, and the speed of the carrier.
A method of direct non vaporizing injection, known as "on column", is reported in U.S. Pat. No. 4,269,608. According to this method, the sample is injected directly into the column, either into the upstream section of the column or into a section of the column, free of stationary phase, connected to the upstream end of the column. The technique requires the sample to be injected while the oven temperature is maintained below the boiling point of the solvent (Sbp). The use of oven temperatures below the boiling point of the solvent, besides needlessly prolonging the analysis, leads to deterioration in the chromatographic peaks, and therefore, deterioration in the results. Further, the use of temperatures much higher than the solvent boiling point might lead to an "explosive" expansion of the solvent in the column, with loss of sample and discrimination such as to render the analysis unreliable. Also, for reasons connected with the preparation of the sample, the solvents used have boiling points lower than 100.degree. C., and therefore, the oven temperatures during injection must be kept very low. The on-column injection method patented by the Applicant (U.S. Pat. No. 4,269,608) allows the sample to be injected at oven temperatures 15.degree.-30.degree. C. above the boiling point of the solvent, eliminating the deformation of the chromatographic peaks, which improves the results. In the embodiment described in the patent, there was a system of secondary cooling, which did not allow injection at oven temperatures very much greater than Sbp.
In the same patent, the Applicant offered a solution to this type of problem: moving the injector and the injection section of the column outside the oven during injection, and returning it after the solvent had passed through the column. The main defect of this solution is that it requires mechanical movement of the injector and column, which, among other things, is difficult to automate.
When the sample contains high boiling point and/or medium volatile compounds and the analysis is specifically for them, the injection at oven temperatures low with respect to the initial temperature required for the chromatographic separation creates needless time losses in heating the oven and cooling after the "temperature program" (the analysis is usually made by programming the oven temperature between an initial and final temperature).